1. Field of the Invention
This invention relates to an image forming apparatus for forming unfixed images, fixing the unfixed images on a recording material and thereby providing desired images. As typical examples of such apparatus, there are printers, electrophotographic copying apparatus, facsimile apparatus, laser beam printers, etc., but in any of these apparatuses, there is provided a fixing rotatable member (a belt, a roller or the like) for fixing unfixed images by heating, heating and pressing or pressure.
2. Description of the Prior Art
Generally, a contact type fixing rotatable member is often used to efficiently fix unfixed images, and at the same time application of a parting agent for preventing offset is requisite.
Taking an ordinary electrophotographic apparatus as an example, the amount of parting agent applied should preferably be slight and uniform for monochromatic images. In the case of multi-colored images where the type of the recording material is varied or the amount of toner constituting the image is increased, it would occur to mind to slow down the fixing speed to improve the fixation, but since the rate at which offset occurs increases, the amount of parting agent applied should preferably be relatively great.
However, applying a relatively great amount of parting agent at all times may result in wasteful loss of the parting agent during non-fixation and therefore, it is preferable to control the supply of the parting agent so that the parting agent is supplied intermittently.
On the other hand, the intermittent, supply of the parting agent which has heretofore been proposed and practised is effected for a predetermined period in an image forming apparatus wherein the fixing speed is constant. Conversely, in most of the conventional apparatuses, the supply the parting agent is continued at all times.
However, in a case where a relatively great amount of parting agent is required as described above, if the fixing speed or the fixing effectiveness is varied for other purposes, for example, in a fixing device provided with fixing rollers, the time required for the parting agent to come from the location at which the parting agent is applied to the location at which the rollers contact the recording material to fix the image thereon, i.e., the nip between the rollers, will differ. Also, the circumferential length of the roller in the area of the roller surface to which the parting agent is applied is varied by the speed difference and the dimensions of that area are varied. Because of these problems, the area of the roller surface to which the parting agent is actually applied has not become a desired one. Further, because of the difference in the arrival time of the parting agent, there occurs a deviation between the period when the area of the roller surface to which the parting agent has been applied passes through the nip and the period when the recording material passes through the nip. Also, the dimensions of the area of the roller surface to which the parting agent has been applied are varied and particularly, if a second fixing speed is lower than a first fixing speed, the dimensions of said area are reduced and cannot cover the size of the recording material.
Thus, in the conventional image forming apparatus, if the fixing speed is changed, the roller surface to which the parting agent is not applied may sometimes be brought into contact with the toner on the recording material, and this may give rise to the problem of offset. Also, the parting agent on the area of the roller surface which has not contacted the recording material is wasted, and this is not only uneconomical but also may cause the environment to be stained. Further, if the parting agent is not supplied to the roller surface corresponding to the leading end portion of the recording material, the recording material may twine around the roller surface, thus causing jam.
These problems were not known because an apparatus in which the fixing speed was varied and the application of the parting agent was intermittently inventor has now recognized these problems.